The present invention relates to a short-arc high-pressure mercury vapor discharge lamp including a pair of discharge electrodes opposed to each other in an arc tube and enclosing mercury and a rare gas in the arc tube. The invention further relates to a lamp unit provided with such a high-pressure mercury vapor discharge lamp.
High-pressure mercury vapor discharge lamps have the advantage of high luminance and therefore, combined with reflectors (for example, parabolic mirrors), are used as the light source for liquid crystal projectors, and so forth. With a trend toward larger screen sizes and higher resolution images, particularly in recent liquid crystal projectors, there has been a demand for lamps that achieve a higher illuminance on the projection screen. In order to obtain such a lamp, it is required to shorten the arc length (distance between the electrodes) and to increase luminous flux by increasing lamp power (rated power and input power).
The above-described shortening of the arc length is required so that light emitted by the lamp reaches the target (the projection screen) with minimal loss. In other words, the closer the light emitting portion (arc) of the lamp is to a point light source, the more loss of converging light caused by the optical system, such as a reflector, can be reduced (i.e., the optical efficiency is improved). More specifically, the luminous flux per unit arc length "PHgr"/d, where "PHgr" (lm) is the luminous flux and d (mm) is the arc length, is equivalent to the arc luminance L (cd/m2), and this arc luminance L determines the screen illuminance on the projector during projection.
An example of a so-called short-arc lamp, in which the above-described shortening of the arc length is attempted, is disclosed in, for example, Japanese Unexamined Patent Publication No. 2-148561. This lamp is a high-pressure mercury vapor discharge lamp in which the lamp power is set at 30 to 50 W and the arc length is set at 1.0 to 1.2 mm. The above-described arc length is very short as compared to, for example, a 40 W high-pressure mercury vapor discharge lamp for general-purpose illumination (HF40 available from Matsushita Electric Industrial Co., Ltd.) which has an arc length of 12 mm. That is to say, this kind of lamp is distinguished from lamps for general purpose illumination and the like in that it normally has an arc length of about 2 mm or less or at the longest about 3 mm or less. Therefore, in the present invention, an arc having an arc length of 3 mm or less is referred to as xe2x80x9cshort arc.xe2x80x9d
The above-described increase in lamp power may be achieved by increasing lamp current or by increasing lamp voltage. However, for a drive circuit that drives a lamp, it is generally easier to increase output voltage than it is to increase current capacity. In addition, when the lamp current is increased, a rise in the temperature of the electrodes is effected by increased Joule loss to the electrodes, and as a result, blackening, caused by vaporization of the electrodes and deposition of this vapor on the inner wall of the arc tube, is more likely to occur. For these reasons, in conventional high-pressure mercury vapor discharge lamps, various methods have been proposed for achieving an increased lamp power by increasing lamp voltage. For example, in a lamp disclosed in the above-described Japanese Unexamined Patent Publication No. 2-148561, by increasing the amount of mercury enclosed and/or by increasing the tube wall loading (lamp power/internal surface area of the arc tube (W/mm2)), the operating pressure of the lamp can be set as high as 200 to 300 atm., to achieve a lamp voltage of 76 to 92 V. In this case, a lamp power of 30 to 50 W is achieved with a lamp current of about 0.33 to 0.66 A. (It should be noted that the lamp power can be easily increased by lengthening the arc length; however, this leads to a reduction in optical efficiency as described above, and therefore it is not possible to achieve a screen illuminance that improves according to the degree of increase in lamp power.)
However, in conventional high-pressure mercury vapor discharge lamps in which the lamp power is increased by increasing the operating pressure and the like as described above, it is difficult to substantially increase lamp power due to a limitation on the strength of the arc tube to withstand pressure.
In view of the foregoing problems, it is an object of the present invention to provide a high-pressure mercury vapor discharge lamp which has a short arc length and achieves a high luminous flux by substantially increasing lamp power.
It is another object of the invention to provide a lamp unit utilizing such a high-pressure mercury vapor discharge lamp.
In order to achieve the above-described substantial increase in lamp power, first, the present inventors tried increasing lamp voltage. However, although the operating pressure of a lamp varies by the size, shape, or the like of the arc tube, the highest possible operating pressure is about 400 atm. In addition, while the operating pressure of a lamp is proportional to the amount of mercury enclosed, the lamp voltage is proportional to only about xc2xd power of the amount of mercury enclosed (Elenbaas, xe2x80x9cThe High Pressure Mercury Vapour Discharge,xe2x80x9d North-Holland Publishing Company, 1951, p30). For this reason, it was difficult to increase lamp voltage to about 90 V or higher, and thus impossible to substantially increase lamp power to, for example, about 125 W or higher. As a result, the highest achievable light output was about 60 (lm/W). (It should be noted that a limitation on the strength of the arc tube to withstand pressure such as described above is due to the limitations of the sealing technique. However, improvement in sealing technique, by which a substantial increase in strength to withstand pressure can be obtained, is not easily achieved, as there are still many technical problems to overcome.)
Further, since it was difficult to increase lamp voltage above the above-described level, the present inventors thought of another technique wherein after having increased lamp voltage as much as possible, lamp current is increased. In doing so, however, an increase in electrode diameter is required in order to reduce Joule loss to the electrodes such as was described above and prevent blackening of the arc tube. However, an increase in electrode diameter increases the area of contact between the sealing portions and the electrodes in the arc tube, causing very small cracks or gaps to be more likely to occur. In other words, because the strength of the sealing portions is reduced, the probability of damage to the arc tube is increased. Hence, in this case also, because there was a limitation on the strength of the arc tube to withstand pressure, it was not possible to substantially increase the lamp current (specifically, above about 1 A, for example), and therefore it was difficult to substantially increase lamp power.
At this point, in order to achieve a substantial increase in lamp power, further and various studies were carried out. From these studies, it was found that the limitations of the drive circuit in terms of an increase in the lamp current such as described above have technically nothing to with the drive circuit itself. In addition, the problem of the strength of the arc tube to withstand pressure, which accompanies an increase in electrode diameter intended to prevent blackening of the arc tube, in fact is due to the approach adopted as described above in which the operating pressure is increased so as to increase the lamp voltage. Thus, the present inventors came up with an approach in which the lamp power may be increased by increasing the lamp current while allowing for a reduction in the lamp voltage.
In other words, power is basically the product of current and voltage, and thus, electrically speaking, an increase in power is equivalent to an increase in voltage and an increase in current. However, in actual high-pressure mercury vapor discharge lamps, when the operating pressure is restricted to a low pressure while allowing for a reduction in the lamp voltage, the limitation on the strength of the arc tube to withstand pressure is reduced. Thus, the electrode diameter may be easily increased to such a level that the arc tube is not damaged and further blackening of the arc tube can be prevented. As a result, the present inventors have found that it is possible to increase the lamp current to such a level that the reduction in the lamp voltage can be sufficiently compensated, and therefore a much higher lamp power than that of conventional lamps is achievable. Thus, the present invention was accomplished.
The foregoing objects are accomplished in accordance with the present invention by providing a short-arc high-pressure mercury vapor discharge lamp comprising a pair of discharge electrodes opposed to each other in an arc tube and enclosing at least mercury and a rare gas in the arc tube, wherein:
the high-pressure mercury vapor discharge lamp may be so constructed as to be operated at a lamp current of 1.5 A or higher, and preferably at 2 A or higher, or to be operated at a lamp voltage/lamp current ratio of approximately 37.5 (V/A) or lower. The above-described short arc here implies an arc having an arc length of 3 mm or less as described above.
By operating the lamp at a high lamp current as described above, a high lamp power is achieved with a relatively low lamp voltage. In addition, because the lamp voltage is relatively low, it is possible to set the operating pressure of the lamp at a low pressure, thereby reducing the limitation on the strength of the arc tube to withstand pressure, and thus the electrode diameter can be easily increased. That is to say, Joule loss can be reduced and the temperature of the electrodes can be restricted to a low temperature by increasing heat conduction, thereby preventing blackening of the arc tube, and thus a longer lamp life is also achieved.
In the above-described high-pressure mercury vapor discharge lamp, the rated power and the internal surface area of the arc tube may be set so that the tube wall loading Pw (Pw=P/Sb) (W/mm2) is 1.0 (W/mm2) or lower, where Sb (mm2) is the internal surface area of the arc tube.
Thus, by setting the tube wall loading to a low value, a high lamp power as described above is achieved, the arc tube is less subject to blackening, and furthermore prevention of damage to the arc tube is ensured.
The above-described high-pressure mercury vapor discharge lamp may be constructed so that the rated power P (W) is such that Pxe2x89xa7125 (W).
In other words, when a lamp is operated at a high lamp current as described above, such a high rated power is attained, and thus a high-pressure mercury vapor discharge lamp that emits a high luminous flux can be obtained.
In the above-described high-pressure mercury vapor discharge lamp, the distance between the electrodes and the rated power may be set so that the rated power per unit arc length P/d (W/mm) is such that P/dxe2x89xa788 (W/mm), where d (mm) is the arc length and P (W) is the rated power.
Thus, because the rated power per unit arc length is sufficiently high, a luminous flux per unit arc length of, for example, 5800 (lm/mm), which is required for a liquid crystal projector, is achieved.
In the above-described high-pressure mercury vapor discharge lamp, the distance between the electrodes, the rated power, the type of fill material, and the amount of fill material may be set so that the luminous flux per unit arc length is 5800 (lm/mm) or higher.
In other words, by operating the lamp at a high lamp current as described above, such a high luminous flux per unit arc length is achieved. Therefore, when, for example, the lamp is utilized in combination with a reflector and so forth, as is the case with a liquid crystal projector, a high optical efficiency and a high luminance are easily achieved.
In the above-described high-pressure mercury vapor discharge lamp, the type of fill material, the amount of fill material, the shape of the arc tube, the cross sectional area in the vicinity of tips of the electrodes, the distance between the electrodes, and the rated power may be set so that the rated power per unit volume of a discharge arc formed between the electrodes Exc2x7j (W/mm3) is such that Exc2x7jxe2x89xa7700 (W/mm3), where E (E=V/d) (V/mm) is the lamp voltage per unit arc length, V (V) being the lamp voltage at stable operation and d (mm) being the arc length, and j (j=I/Se) (A/mm2) is the current density at the tips of the electrodes, I (A) being the lamp current at stable operation and Se (mm2) being the cross sectional area in the vicinity of the tips of the electrodes.
Thus, in this case also, it is possible to increase the rated power per unit arc length with no damage to the arc tube. Consequently, a high luminous flux per unit arc length, for example, a luminous flux per unit arc length of 5800 (lm/mm) as described above, which is required for a liquid crystal projector, is achieved.
The above-described high-pressure mercury vapor discharge lamp may further enclose at least one member selected from the group consisting of a halogen gas, a nonmetallic halide, and a metal halide, in the arc tube.
Thus, the so-called halogen cycle takes place in the arc tube, thereby preventing vaporized electrode material from depositing on the inner wall of the arc tube and preventing reduction in the light transmittance of the wall of the arc tube. Therefore, blackening of the arc tube is controlled, and thus a longer lamp life is achieved.
The present invention further provides a lamp unit comprising:
the above-described high-pressure mercury vapor discharge lamp; and
a reflector for reflecting light emitted by the above-described high-pressure mercury vapor discharge lamp such that the light is converted into a parallel beam, a convergent beam in which light converges to a predetermined micro-area, or a divergent beam which is substantially the same as light diverged from a predetermined micro-area.
Thus, since the arc length is short, a high optical efficiency is achieved. In addition, since the luminous flux per unit arc length is high, it is possible to display brighter images in an image display system, such as a liquid crystal projector.